Extreme Programming

XProgramming.com
Yahoo! XP group

Pragmatic Programmers

The Pragmatic Programmers (Andy Hunt, Dave Thomas, et al.)
Yahoo! Pragmatic Programmers group
Yahoo! DFW Pragmatic Programmers group I've yet to attend a meeting

Other Yahoo! groups I read

Refactoring
Test Driven Development (TDD)
Fitnesse (front end to Ward Cunningham's FIT)